One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The usual way of restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient's chest. The switching mechanism used in most contemporary external defibrillators is a high-energy transfer relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
Although the high-energy transfer relays used in external cardiac defibrillators have performed satisfactorily, they have a variety of disadvantages. One of the major disadvantages is the electromagnetic interference (EMI) that is caused when the relay is closed. EMI can be detrimental to the signals used by nearby control circuits and makes the use of EMI-sensitive circuitry impractical during the application of the defibrillation pulse. Due to the EMI interference, external defibrillators typically temporarily place all control circuitry in an "inactive" state while a defibrillation pulse is applied. External defibrillators are therefore unable to verify that the switching mechanism or relay is working properly because a limited amount of circuitry is operational during the application of the defibrillation pulse.
An additional disadvantage of using a relay is that prior to the application of the defibrillation pulse, it may be impractical to test the integrity of the relay. For example, one method for testing the relay requires discharging the energy storage capacitor into a test load. This and similar methods require not only discharging most of the energy in the energy storage capacitor during each test, but also require extra circuitry including a test load.
The present invention is directed to providing a method and apparatus that overcome the foregoing and other disadvantages. More specifically, the present invention is directed to providing a method and apparatus for verifying the integrity of an output circuit before and during the application of a defibrillation pulse.